known physical processes. Some molecular and morphological structures form only by biological processes (cholesterol, dinosaur skulls), while others clearly relate to physical processes (large quartz crystals, for example), and still others exist in a zone of overlap (2-micron spheres, amino acids). We can never eliminate the zone of overlap, but better understanding of the products of both biological and physical processes will better equip us to pursue questions of life’s antiquity on Earth and its distribution through the Solar System.
Our understanding of our own origins remains sketchy, but it is expanding at an accelerating pace. Thanks to contributions from many fields and approaches, scientists are better prepared to approach a truly tantalizing question: Are we alone, or has life also evolved elsewhere? If life exists elsewhere, what forms does it take? With continuing planetary exploration, Earth scientists will be able to establish with greater certainty whether life could have originated elsewhere in our Solar System—and even whether organisms could have become established on Earth by meteoritic transfer from another planet. Thanks to discoveries of the National Aeronautics and Space Administration’s rover Opportunity, we now know that around the time life took root on Earth, at least regional environments on Mars’ surface were episodically wet (Knoll et al., 2005). But they were also oxidizing and strongly acidic—serious obstacles to many of the prebiotic chemical pathways thought to have been important on Earth. Was early Mars arid, oxidizing, and acidic globally or just regionally, and when were such environments established? Clay minerals in some of Mars’ oldest terrains may signal that early in its history our neighbor was relatively wet but less acidic (Bibring et al., 2006). Also, carbonate and sulfide minerals precipitated from fluids flowing through crustal fractures document at least transient subterranean environments neither strongly acidic nor oxidizing (McKay et al., 1996). Only further exploration, with Earth and planetary scientists working in partnership, will establish whether life on Earth is unique in our Solar System or merely uniquely successful.
While synthetic organic chemistry and molecular biology will continue to provide the experimental basis for probing life’s origins, Earth scientists will increasingly specify the conditions under which laboratory experiments are run. Stratigraphers, paleontologists, biogeochemists, and geochronologists can provide sharper constraints on when life arose and the metabolic character of early organisms. Geochemists focused on both crustal differentiation and low-temperature reactions can build an improved sense of redox conditions, weathering reactions, and metal abundances on the early Earth. Modelers can use new data to provide more sophisticated hypotheses about how our planetary surface operated in its infancy, setting the stage for the intercalation of biological processes into the Earth system. And planetary scientists, now exploring Mars and other bodies at a resolution previously limited to Earth, can provide comparable insights about environmental (and, at least potentially, biological) evolution on other planets.